Pneumococcal surface proteins: when the whole is greater than the sum of its parts

Surface-exposed proteins of pathogenic bacteria are considered as potential virulence factors through their direct contribution to host-pathogen interactions. Four families of surface proteins decorate the cell surface of the human pathogen Streptococcus pneumoniae. Besides lipoproteins and LPXTG proteins, also present in other gram-positive bacteria, the pneumococcus presents the choline-binding protein (CBP) family and the non-classical surface proteins (NCSPs). The CBPs present specific structural features that allow their anchorage to the cell envelope through non-covalent interaction with choline residues of lipoteichoic acid and teichoic acid. NCSP is an umbrella term for less characterized proteins displaying moonlighting functions on the pneumococcal surface that lack a leader peptide and membrane-anchor motif. Considering the unceasing evolution of microbial species under the selective pressure of antibiotic use, detailed understanding of the interaction between pathogen and the host cells is required for the development of novel therapeutic strategies to combat pneumococcal infections. This article reviews recent progress in the investigation of the three-dimensional structures of surface-exposed pneumococcal proteins. The modular nature of some of them produces a great versatility and sophistication of the virulence functions that, in most cases, cannot be deduced by the structural analysis of the isolated modules.